Method and apparatus of transmitting a spatial stream for MU-MIMO in a wireless local area network system

ABSTRACT

A method of transmitting a spatial stream for multi user (MU)-multiple input multiple output (MIMO) in a wireless local area network system and a transmitter for performing the method are provided. The method includes transmitting, to a receiver, a management frame including group information to assign or change a position of a plurality of spatial streams corresponding to each of a plurality of groups, and transmitting, to the receiver, a frame including at least one spatial stream, wherein the group information includes a plurality of group indicators and a plurality of spatial stream (SS) indicators, each of the plurality of group indicators indicating whether the receiver is a member of each of the plurality of groups, each of the plurality of SS indicators indicating a position of the plurality of spatial streams corresponding to each of the plurality of groups.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/016,900, filed on Jan. 28, 2011, now U.S. Pat. No. 8,891,666, whichclaims the benefit of U.S. Provisional Application No. 61/299,353, filedon Jan. 29, 2010, 61/317,697, filed on Mar. 26, 2010, 61/327,716 filedon Apr. 25, 2010, and 61/362,282, filed on Jul. 7, 2010, the contents ofall of which are hereby incorporated by reference herein in theirentireties.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to wireless communication, and moreparticularly, to a method and apparatus of transmitting a spatial streamfor MU-MIMO in a wireless local area network system.

Related Art

With the recent development of information communication technology, avariety of wireless communication techniques are being developed. Fromamong them, a WLAN is a technique which enables wireless access to theInternet at home or companies or in a specific service providing areausing mobile terminals, such as a Personal Digital Assistant (PDA), alaptop computer, and a Portable Multimedia Player (PMP), on the basis ofradio frequency technology.

Since Institute of Electrical and Electronics Engineers (IEEE) 802(i.e., the standard organization of WLAN technology) has been set up onFebruary, 1980, lots of standardization tasks are being performed.

The initial WLAN technology was able to support the bit rate of 1 to 2Mbps through frequency hopping, band spreading, and infraredcommunication using a 2.4 GHz frequency band in accordance with IEEE802.11, but the recent WLAN technology can support the maximum bit rateof 54 Mbps using Orthogonal Frequency Division Multiplex (OFDM). Inaddition, in the IEEE 802.11, the standardization of various techniques,such as the improvements of Quality of Service (QoS), the compatibilityof Access Point (AP) protocols, security enhancement, radio resourcemeasurement, wireless access vehicular environment for vehicleenvironments, fast roaming, a mesh network, interworking with anexternal network, and wireless network management, is put to practicaluse or being developed.

IEEE 802.11b of the IEEE 802.11 supports a maximum transmission speed of11 Mbs while using the 2.4 GHz frequency band. IEEE 802.11acommercialized after the IEEE 802.11b has reduced the influence ofinterference as compared with the very complicated 2.4 GHz frequencyband by using a 5 GHz frequency band not the 2.4 GHz frequency band andalso improved the transmission speed up to a maximum of 54 Mbps usingthe OFDM technique. However, the IEEE 802.11a is disadvantageous in thatthe communication distance is shorter than that of the IEEE 802.11b.Further, IEEE 802.11g implements a maximum communication speed of 54Mbps using the 2.4 GHz frequency band like the IEEE 802.11b andsatisfies backward compatibility. The IEEE 802.11g is being in thespotlight and superior to the IEEE 802.11a even in the communicationdistance.

Further, as a technique for overcoming limits to the communication speedpointed out as vulnerabilities in the WLAN, there is IEEE 802.11n whichhas recently been standardized. The IEEE 802.11n has its object toincrease the speed and reliability of a network and to expand theoperating distance of a wireless network. More particularly, the IEEE802.11n is based on a Multiple Inputs and Multiple Outputs (MIMO)technique using multiple antennas on both sides of a transmitter and areceiver in order to support a High Throughput (HT) having a dataprocessing speed of 540 Mbps or higher, minimize transmission error, andoptimize the data rate. Further, the IEEE 802.11n may use not only acoding method of transmitting several redundant copies in order toincrease data reliability, but also an OFDM (Orthogonal FrequencyDivision Multiplex) method in order to increase the data rate.

With the WLAN being widely spread and applications using the WLANbecoming diverse, a need for a new WLAN system capable of supporting ahigher throughput than the data processing speed supported by the IEEE802.11n is recently gathering strength. A Very High Throughput (VHT)WLAN system is one of IEEE 802.11 WLAN systems which have recently beenproposed in order to support a data processing speed of 1 Gbps orhigher. The name ‘VHT WLAN system’ is arbitrary. A feasibility test fora system using 8×8 MIMO and a channel bandwidth of 80 MHz or higher soas to provide the throughput of 1 Gbps or higher is in progress.

When implemented is a method for transmitting data to multiple STAsbelonging to 802.11ac VHT WLAN system supporting the MU-MIMOtransmission, the STAs must be informed through the VHT-SIG part of PLCPpreamble that which STA receives data through which spatial stream.However, the association ID intended for identifying each individual STArequires a considerable number of bits; accordingly, a large amount ofbits are required to inform the multiple STAs of spatial streaminformation. Therefore, one should take account of a method for reducingthe number of bits carried by the PLCP preamble and informing STAs ofthe number of spatial streams.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus of transmitting aspatial stream for MU-MIMO in a wireless local area network system.

In an aspect, a method of transmitting a spatial stream for multi user(MU)-multiple input multiple output (MIMO) in a wireless local areanetwork system, performed by a transmitter, comprises transmitting, to areceiver, a management frame including group information to assign orchange a position of a spatial stream corresponding to each of aplurality of groups, and transmitting, to the receiver, a frameincluding at least one spatial stream, wherein the group informationincludes a plurality of group indicators and a plurality of spatialstream (SS) indicators, each of the plurality of group indicatorsindicating whether or not the receiver is a member of each of theplurality of groups, each of the plurality of SS indicators indicating aposition of a spatial stream corresponding to each of the plurality ofgroups.

The frame further may include a header including a group identity fieldindicating a membership group of the plurality of groups, wherein thereceiver is a member of the membership group.

If the group identity field is set to a predefined value, the frame maybe transmitted by using single user (SU)-MIMO.

The header further may include a long training field used to estimatechannel for decoding the at least one spatial stream.

Each of the plurality of SS indicators may indicate the position of thespatial stream within four spatial streams.

The transmitter may be an access point (AP).

The position of the spatial stream corresponding to each of theplurality of groups may be associated with one or more receivers.

In another aspect, a transmitter for MU-MIMO in a wireless local areanetwork system comprises a processor and a radio frequency (RF) unitoperatively coupled to the processor and configured to transmit a frame,wherein the processor is configured to transmit, to a receiver, amanagement frame including group information to assign or change aposition of a spatial stream corresponding to each of a plurality ofgroups and transmitting, to the receiver, a frame including at least onespatial stream, wherein the group information includes a plurality ofgroup indicators and a plurality of SS indicators, each of the pluralityof group indicators indicating whether or not the receiver is a memberof each of the plurality of groups, each of the plurality of SSindicators indicating a position of a spatial stream corresponding toeach of the plurality of groups.

The frame may further include a header including a group identity fieldindicating a membership group of the plurality of groups, wherein thereceiver is a member of the membership group.

If the group identity field is set to a predefined value, the frame maybe transmitted by using SU-MIMO.

The header may further include a long training field used to estimatechannel for decoding the at least one spatial stream.

Each of the plurality of SS indicators may indicate the position of thespatial stream within four spatial streams.

In still another aspect, a method of receiving a spatial stream forMU-MIMO in a wireless local area network system, performed by areceiver, comprises receiving, from a transmitter, a management frameincluding group information to assign or change a position of a spatialstream corresponding to each of a plurality of groups, receiving, fromthe transmitter, a header in a frame, identifying a membership groupbased on the header, and if the receiver is a member of the membershipgroup, receiving, from the receiver, at least one spatial stream in theframe, wherein the group information includes a plurality of groupindicators and a plurality of SS indicators, each of the plurality ofgroup indicators indicating whether or not the receiver is a member ofeach of the plurality of groups, each of the plurality of SS indicatorsindicating a position of a spatial stream corresponding to each of theplurality of groups, wherein the header includes a group identity fieldindicating the membership group of the plurality of groups.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing an example of a WLAN system.

FIG. 2 is one example of a PLCP frame format supporting the IEEE802.11n.

FIG. 3 is a block diagram illustrating one example of a PLCP frameformat according to an embodiment of the present invention.

FIG. 4 illustrates a procedure for transmitting and receiving framesaccording to the embodiment of the present invention.

FIG. 5 is one example of indicating that the value of GGIF andparticular STAs are associated logically with each other in themanagement frame.

FIGS. 6 to 10 illustrate one example of a method for providing STAs withgroup ID indication information and position indication information ofthe STAs through the management frame.

FIG. 11 illustrates one example of configuration of Group IdentityIndication Field (GIIF) and Spatial Stream Association ID Field (SSAIF)for MU-MIMO transmission according to another embodiment of the presentinvention.

FIG. 12 illustrates one example of indicating STA group sets in the PHYlayer and indicating STA groups in the MAC layer according to anembodiment of the present invention.

FIG. 13 is one example of frame transmission by using Group Setindication signaled in the PHY layer and Group indication signaled inthe MAC layer.

FIG. 14 illustrates a format of the management information included inthe management frame according to an embodiment of the presentinvention.

FIG. 15 is an example where a method of indicating a group by using MAClayer and PHY layer is applied to data packet transmission.

FIG. 16 is a block diagram illustrating a transmitter in which oneembodiment of the present invention is implemented.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, some embodiments of the present invention are described indetail with reference to the accompanying drawings. The followingembodiments can be usefully applied to a Very High Throughput (VHT) WLANsystem using a channel bandwidth of 80 MHz, but not limited thereto. Forexample, the embodiments of the present invention can also be applied toa WLAN system using a channel bandwidth, exceeding 40 MHz or 80 MHz andincluding a plurality of channel blocks.

FIG. 1 is a diagram showing an example of a WLAN system to which anembodiment of the present invention can be applied. The WLAN systemaccording to the example of FIG. 1 is a Very High Throughput (VHT) WLANsystem.

Referring to FIG. 1, the WLAN system, such as a VHT WLAN system,includes one or more Basic Service Sets (hereinafter referred to as a‘BSS’). The BSS is a set of STAtions (hereinafter referred to as an‘STA’) which can communicate with each other through successfulsynchronization. The BSS is not a concept indicating a specific area.Further, as in a WLAN system to which an embodiment of the presentinvention can be applied, a BSS supporting ultra-high data processing of1 GHz or higher at the MAC Service Access Point (SAP) is referred to asa VHT BSS.

The VHT BSS can be classified into an infrastructure BSS and anIndependent BSS (hereinafter referred to as an ‘IBSS’). Aninfrastructure BSS is shown in FIG. 1. The infrastructure BSS BSS1 andBSS2 includes one or more Non-AP STAs STA 1, STA 3, and STA 4, AccessPoints AP 1 (STA 2) and AP 2 (STA 5) providing distribution service, anda Distribution System (hereinafter referred to as a ‘DS’)interconnecting the plurality of APs AP 1 and AP 2. In theinfrastructure BSS, an AP STA manages the Non-AP STAs of the BSS.

On the other hand, the IBSS (i.e., the independent BSS) is a BSSoperating in the ad-hoc mode. The IBSS does not include a centralizedmanagement entity, performing a management function at the center,because it does not include an AP VHT STA. That is, in the IBSS, Non-APSTAs are managed in a distributed manner. Further, in the IBSS, all STAscan be composed of mobile STAs, and they form a self-contained networkbecause access to a DS is not permitted.

An STA includes both an AP (i.e., in a broad sense) and a Non-AP STAwhich are certain function media, including a Medium Access Control(MAC) layer and a physical layer interface for a radio medium inaccordance with the IEEE 802.11 standard. Further, in a multi-channelenvironment to be described later, an STA supporting ultra-high dataprocessing of 1 GHz or higher is referred to as a VHT STA. In a VHT WLANsystem to which an embodiment of the present invention can be applied,all STAs included in the above BSS can be VHT STAs, or VHT STAs andlegacy STAs (e.g., HT STAs in accordance with the IEEE 802.11n standard)can coexist in the STAs included in the above BSS.

An STA for wireless communication includes a processor and a transceiverand further includes a user interface, display means, etc. The processoris a function unit configured to generate a frame which will betransmitted over a wireless network or to process a frame received overthe wireless network. The processor performs various functions forcontrolling the STA. Further, the transceiver is functionally connectedto the processor and configured to transmit and receive a frame over thewireless network for the STA.

A portable terminal used by a user, from among STAs, corresponds to aNon-AP STA (e.g., STA1, STA3 and STA4). If an STA can be simply referredas a Non-AP STA. The Non-AP STA may also be referred to as anotherterminology, such as a terminal, a Wireless Transmit/Receive Unit(WTRU), a User Equipment (UE), a Mobile Station (MS), a mobile terminal,or a mobile subscriber unit. Further, in a multi-channel environment tobe described later, a Non-AP STA supporting ultra-high data processingof 1 GHz or higher is referred to as a Non-AP VHT STA or simply a VHTSTA.

Further, the APs AP1 and AP2 are function entities providing access tothe DS via a radio medium for STAs (i.e., association STAs) associatedtherewith. In an infrastructure BSS including an AP, communicationbetween Non-AP STAs is performed via the AP in principle. In case wherea direct link is set up, communication can be directly performed betweenNon-AP STAs. The AP may also be referred to as a concentratedcontroller, a Base Station (BS), a node-B, a Base Transceiver System(BTS), or a site controller. Further, in a multi-channel environment tobe described later, an AP supporting ultra-high data processing of 1 GHzor higher is referred to as a VHT AP.

A plurality of infrastructure BSSs can be interconnected through a DS(Distribution System). The plurality of BSSs interconnected through theDS is called an Extended Service Set (ESS). STAs included in the ESS cancommunicate with each other. Non-AP STAs can continue to communicatewith each other within the same ESS and move from one BSS to anotherBSS.

The DS is a mechanism for enabling one AP to communicate with anotherAP. According to the mechanism, an AP can transmit a frame to STAs whichare managed by the AP and connected to a BSS, can transfer a frame toany one STA in case where the STA has moved to another BSS, or cantransfer a frame over an external network, such as a wired network. TheDS needs not to be necessarily a network, and it can have any type aslong as it can provide predetermined distribution service regulated inthe IEEE 802.11 standard. For example, the DS may be a wireless network,such as a mesh network, or a physical structure for interconnecting APs.

FIG. 2 is one example of a PLCP frame format supporting the IEEE802.11n.

The IEEE 802.11n High Throughput (hereinafter, it is called an HTsystem) specifications support the PLCP format that supports the legacy802.11a, 802.11b, and 802.11g. The PLCP format 210 supporting legacySTAs (Non-HT STAs) is transmitted in the order of legacy short trainingfield (L-STF), legacy long training field (L-LTF), legacy signal(L-SIG), and data. L-STF is used for frame timing acquisition andautomatic gain control convergence, while L-LTF for carrying out channelestimation to demodulate the L-SIG and the data. The L-SIG contains theinformation for demodulating and decoding the data subsequent to thePLCP.

Meanwhile, a system consisting of HT STAs only makes use of HT-GreenField format 220, a PLCP format optimized for the HT STA. The HT-GreenField PLCP format 220 is transmitted in the order of HT green fieldshort training field (HT-GF-STF), HT long training field (HT-LTF), HTsignal (HT-SIG), and data. The HT-GF-STF is used for frame timingacquisition and automatic gain control convergence, while the HT-LTF forcarrying out channel estimation to demodulate the HT-SIG and the data.The HT-SIG contains the information for demodulating and decoding thedata subsequent to the PLCP.

In addition, a system where legacy stations (Non-HT STA) and HT STAsco-exist supports an HT-mixed format 230, a PLCP format designed tosupport HT. In the HT-mixed format 230, L-STF, L-LTF, and L-SIG arefirst transmitted to allow Non-HT STAs to recognize the format. Next,transmitted is the HT signal (HT-SIG) transmitting information necessaryfor demodulating and decoding the data transmitted to an HT STA. Fielddata up to the HT-SIG are transmitted without using a beam-formingtechnique in order for various STAs including legacy systems to receiveinformation, while transmission of the HT-LTF and the data to betransmitted after the HT-SIG is carried out by applying signaltransmission through precoding. At this time, by taking account of thepower variation due to the precoding at the STAs which receive theprecoded signal, HT short training field (HT-STF) is transmitted, afterwhich the HT-LTFs and the data are transmitted.

To utilize given channels efficiently in an IEEE 802.11 system, it isrequired that using of MU-MIMO type transmission by scheduling aplurality of STAs simultaneously. MU-MIMO is a technique where multipleSTAs, each with potentially multiple antennas, transmit and/or receiveindependent data streams simultaneously. To support the MU-MIMO, thecorresponding STAs should be informed of the fact that the data aretransmitted through a particular spatial stream and subsequently, thecorresponding STAs should be prepared to receive actual data accordingto the spatial stream.

FIG. 3 is a block diagram illustrating one example of a PLCP frameformat according to an embodiment of the present invention.

The VHT-mixed format 300 of FIG. 3 is basically the same as the HT-mixedformat 230 of FIG. 2. In other words, L-STF, L-LTF, and L-SIG are firsttransmitted to allow Non-HT STAs to recognize the PLCP frame.Afterwards, VHT-SIG field containing control information for VHT STAs istransmitted.

According to an embodiment of the present invention, the VHT-SIG fieldincludes a group ID indicator and a spatial stream (SS) indicator ascontrol information. The VHT-SIG field may be transmitted while beingdivided into VHT-SIG-A containing common information about all the VHTSTAs and VHT-SIG-B containing control information for separate VHT STAs.At this time, during the MU-MIMO transmission, the group ID indicatorand the spatial stream (SS) indicator may be included in the VHT-SIG-A.

It is necessary for a transmitter attempting the MU-MIMO transmission toinform a plurality of receivers of particular spatial streams toreceive. In other words, it is required for the transmitter to indicatethrough which spatial stream (SS) data are transmitted to each receiver,thereby preparing the receiver to receive the corresponding SS. At thistime, the transmitter can be an AP, while the plurality of receivers canbe the target STAs of DL MU-MIMO transmission. The target STAs of DLMU-MIMO transmission can be expressed by MU-MIMO paired STAs, recipientsof MU-MIMO transmission, and the like. In what follows, for theconvenience of description, it is assumed that DL MU-MIMO transmissionis performed, where the AP transmits data to multiple STAs throughMU-MIMO transmission.

In the VHT-SIG-A contained in the PLCP header of a PPDU (PLCP protocoldata unit) transmitted through MU-MIMO transmission, a Group ID fieldcan be included. The Group ID field indicates the recipients of thePPDU. The AP can transmit a management frame before sending MU-MIMO datapackets through the MU-MIMO transmission. The management frame is aframe transmitted to assign or change positions of the STAscorresponding to the respective groups to which the target STAs belong.

The management frame can include group definition information. The groupdefinition information includes information indicating one or moregroups to which each individual STA belongs for the STAs that can be thepotential recipients of MU-MIMO transmission and the positioninformation of plurality of spatial streams assigned to the STAcorresponding to each individual group. At this time, the positioninformation of the spatial streams can be regarded as the information ofa spatial stream set assigned to an STA when the STA receives MU-MIMOdata packets as a member of a particular group. Wherein a spatial streamset contains plurality of spatial streams. The position information of aspatial stream set may be regarded as indication information of thespatial stream. In view of an STA, if the STA has multiple group IDs, inother words, the STA becomes a member of multiple groups, the positioninformation of a spatial stream set corresponds to the informationindicating a spatial stream set assigned to the STA in the individualgroup to which the STA can belong. The corresponding STA can identifythe spatial stream set through which the data for the STA aretransmitted by utilizing the position information while receivingMU-MIMO data packets transmitted to the group to which the STA belongs.

To be more specific, the group definition information may includeinformation indicating a group to which each individual STA belongs anda group to which the STA does not belong. In the group definitioninformation, how the AP informs of the group information related to STAscan be either directly informing each individual STA of thecorresponding group to which the STA belongs or informing eachindividual group of which STA belongs to the group. The group definitioninformation can further include the information indicating the positionof the spatial stream for a particular STA among the total spatialstreams transmitted through the MU-MIMO transmission.

In other words, the management frame includes the information indicatingwhich STA belongs to which group and the information indicating theposition of a spatial stream in the MU-MIMO transmission correspondingto each group. The management frame may be transmitted for eachindividual STA. Each individual STA gets to know which group it belongsto by receiving the management frame and the position of a spatialstream assigned to itself in the group. When an STA receives MU-MIMOdata packets, it checks whether the data packets are those transmittedto the group to which it belongs through the group ID field included inthe VHT SIG of the MU-MIMO data packet. If it is found that the datapackets are intended for the group to which the STA belongs, the STA canthen determine the data stream transmitted to itself through theposition information in the corresponding group. In other words, thespatial stream set in question through which the data directed to theSTA are transmitted is determined based on the group ID and the positionof the STA in the group indicated by the group ID and the STA can choosethe spatial stream that is supposed to be received by the STA.

FIG. 4 illustrates a procedure for transmitting and receiving framesaccording to the embodiment of the present invention.

The example of FIG. 4 illustrates a case where the AP, as a transmitter,transmits data packets 420 to STA #1 to STA # N. The AP transmits aGroup ID management frame 410 before sending data packets 420 to STA #1to STA # N. As described above, the Group ID management frame 410includes group indication information and spatial stream indicationinformation. The Group ID management frame 410 can be transmitted toeach individual STA through unicast transmission.

After transmission of the Group ID management frame 410, the AP cantransmit MU-MIMO data packets. The AP, to carry out MU-MIMOtransmission, can perform MU-MIMO primitive transmission notifying theMU-MIMO transmission and a sounding procedure for channel estimation.

The PLCP header of the MU-MIMO data packet 420 can include the Group IDindication information and the spatial stream indication information.Each of STA #1 to STA # N reads the Group ID indication informationincluded in the PLCP header of the data packet 420 and checks whetherthe data packet is transmitted to itself; if the data packet is found tobe directed to the group to which the STA belongs, the STA, according tothe spatial stream indication information assigned to itself from thecorresponding group, can receive the spatial stream through which theSTA's data are transmitted.

At this time, the Group ID indication information of the PLCP header ofthe data packet 420 can indicate transmitting data packets 420 throughSU-MIMO transmission to a particular STA rather than to a particulargroup. In other words, if the Group ID indication information has thelength of M bits, the Group ID indication information can indicate 2^(M)states. That is to say, if the states are all used for indicatinggroups. 2^(M) groups can be indicated. However, it may not necessarilybe required to use all of the 2^(M) states for indicating groups sincethe number of actual operating groups may be less than 2^(M). Therefore,a part of 2^(M) states can be used for indicating SU-MIMO transmissionrather than indicating Group IDs. As one example, in the case when theGroup ID indication information is transmitted through the Group IDfield having a length of 6 bits in the PLCP header, 63 states out of2⁶=64 states available can be assigned for indicating particular groups,while the remaining one state can be used for indicating SU-MIMOtransmission or indicating data packets broadcast.

The group definition information transmitted to a particular STA by theAP (information about one or more groups to which the corresponding STAbelongs and the position information of the corresponding STA in theeach of the corresponding groups) can be transmitted to STAs whileincluded in the management frame with various forms. In a specificmethod of transmitting the group definition information described below,the name, the form by which it is included in the PPDU, the position(e.g., it is transmitted while being included in the VHT-SIG-A), and theorder of transmission are just introduced for an illustration purpose;they can also be implemented by a combination of various embodimentsdescribed in the following.

In what follows, various examples of transmitting the group definitioninformation to STAs through the management frame, more specifically,transmitting the group information of the corresponding STA and theposition information of the corresponding STA in each individual groupare described.

In one embodiment of the present invention, the AP can inform STAs ofthe data spatial stream configuration through MU-MIMO transmission byusing the group definition information. At this time, the groupdefinition information transmitted while being included in themanagement frame can include Group ID Indicator (GGI) and Spatial StreamAssociation Indicator (SSAI). The GGI is the information for indicatingSTAs that are supposed to receive data through MU-MIMO transmission,while the SSAI relates to the information about the data spatial streamconfiguration that the corresponding STAs are supposed to receive. Inother words, the GGI is one example of the information for informing anSTA of a group to which the corresponding STA belongs, while the SSAI isone example of the position information of a spatial stream. The GGI andthe SSAI can be transmitted while being included in a field of theVHT-SIG of the PLCP preamble header.

The GGIF (GGI Field) that contains the GGI can have the informationabout which STAs receive the data from the AP through the MU-MIMOtransmission; the STAs can be associated logically with the respectiveGGIF numbers. The AP, before the MU-MIMO transmission, can indicate thevalue of a particular GGIF and STAs associated logically with the valuethrough the management frame.

FIG. 5 is one example of indicating that the value of GGIF andparticular STAs are associated logically with each other in themanagement frame.

According to the example of FIG. 5, a group ID and the association ID(s)of the STA(s) belonging to the corresponding group are transmitted; andthe STA that receives the IDs can know the group to which it belongs. Inother words, the STA that received the management frame can check whichgroup its association ID belongs to and then obtain the ID(s) of one ormore groups to which it belongs.

FIGS. 6 to 10 illustrate one example of a method for providing STAs withgroup ID indication information and position indication information ofthe STAs through the management frame.

FIG. 6 illustrates one example of a method where the AP delivers thegroup ID information for each individual STA and the position indicationinformation of the STA in the corresponding group. In the FIG. 6, theMU-MIMO Group ID field can indicate the Group ID value directly, orinclude the information indicating whether STAs receiving the managementframe are included in the corresponding group. The Spatial StreamAssociation ID transmitted subsequently can include the informationindicating spatial streams associated logically with the Group ID or theposition indication information of the STA in the corresponding group,namely, the information indicating which spatial stream (SS) the STAshould receive when receiving MU-MIMO data packets as a member of thecorresponding group. In the example of FIG. 6, the pair of the MU-MIMOGroup ID Field and the Spatial Stream Association ID can be transmittedas many as the number of groups to which the STAs receiving themanagement frame belong or as many as the total number of groups. Themanagement frame of FIG. 6 can be transmitted to each individual STAthrough unicast transmission. Therefore, multiple STAs can belong to thesame group and the same spatial stream can be assigned to STAs havingdifferent group IDs.

FIG. 7 is an example where STAs are informed of a group ID andassociation IDs of spatial streams assigned to the respective STAsbelonging to the corresponding group. The AP can transmit a particularGroup ID and the Spatial Stream Association ID {0, 1, 2, . . . }corresponding to each Group ID to a plurality of STAs through themanagement frame.

FIG. 8 illustrates one example where the Group ID and the Spatial SSAIDare transmitted in pairs according to the STA Association ID.

The Group ID and the Spatial SSAID are transmitted in pairs according tothe STA Association ID through the management frame broadcast. Differentfrom the example of FIG. 7, multiple STAs can be associated logicallywith a single Spatial Stream Association ID.

FIG. 9 illustrates a case where information about multiple groups,particular group IDs, and STA IDs are associated with each other forparticular STAs. To this end, the STA IDs, multiple Group IDs, and Nstsgroup index can be transmitted through the management frame.

FIG. 10 is another example of a method for informing multiple STAs ofthe same spatial stream association ID. As shown in FIG. 10, the AP, byusing the management frame, transmits STA-AP association IDs of the STAsassociated logically with the Group ID and the Spatial stream ID; andenables the STAs to know the group to which the corresponding STAsbelong and spatial streams assigned to the respective STAs.

In a method of transmitting group indication information, positioninformation of an STA, or indication information of a plurality ofspatial streams to the STA by using the management frame describedabove, when specifying an STA was needed, the Association ID of the STAhas been used. Depending on the needs, however, the MAC ID (MAC address)of the STA can be utilized instead of the Association ID of the STA. Inother words, in the previous example, the Association IDs of the STAscan be replaced with identifiers that enable identifying the STAs. Asshown in the example of FIG. 7, when the management frame is transmittedto each individual STA through unicast transmission, the receiveraddress (RA) of the management frame, namely, the MAC address of the STAcan be regarded to have been used as the indicator of the STA.

In addition, according to an embodiment, the Spatial Stream AssociationID can be expressed as a group index of NSTS indicating a plurality ofspatial streams. In other words, the Spatial Stream Association ID isthe index values of NSTS representing numerated values of the spatialstreams transmitted by the AP, indicating the spatial streams assignedto the STAs. The NSTS group index and the Spatial Stream AssociationIndex can refer to the same field.

The Spatial Stream Association ID (SSAID) enables the STAs to know whichspatial streams are transmitted to them when the STAs receive MU-MIMOdata packets. The previous example illustrates a case where the APinforms the STA of a plurality of spatial streams assigned to thecorresponding STA by using the Spatial Stream Association ID. In anothermethod, as described earlier, the AP transmits the position informationof the STAs in the corresponding group and enables the correspondingSTAs to know which spatial streams to receive.

FIG. 11 illustrates one example of configuration of Group IdentityIndication Field (GIIF) and Spatial Stream Association ID Field (SSAIF)for MU-MIMO transmission according to another embodiment of the presentinvention.

The AP allows each individual STA to know one or more groups to which itbelongs by transmitting logical association between a group ID and eachindividual STA to all the STAs through the management frame; when thegroup ID and STAs are attempted to establish logical association, alogical order of the STAs is determined and informed. At this time, thelogical association between the group ID and the STAs does notnecessarily satisfy a one-to-one relationship; on the contrary, thelogical association can be established in the form of one-to-manycorrespondence.

The SSAIF indicates sequentially in the form of a bitmap how manyspatial streams are used by STAs belonging to a particular group ID. Asa more concrete example, the number of 1s counted from the MSB of theSSAIF indicates the number of spatial streams used by a first STAbelonging to some group ID; the number of 0s from the bit next to theMSB the number of spatial streams used by a second STA, and the numberof is subsequent to the previous bit the number of spatial streams bythe next STA. In this way, by repetition of 1s and 0s, the number ofspatial streams for each individual STA is represented in the form of astring of numerals.

At this time, a first MSB (Most Significant Bit) of a first STA can beomitted. Since no problematic situation occurs if it is already knownthat at least one or more spatial streams are assigned to the first STAeven though the first MSB is omitted for the first STA, SSAIF bit widthcan be compressed by omitting the first MSB for the first STA.

According to another embodiment of the present invention, the value ofthe SSAIF, N_(SS-Field) ^(g) is interpreted differently according to theGroup ID field. According to the embodiment of the present invention,N_(SS-Field) ^(g) represents the number of spatial streams of STAsbelonging to each group. The Group ID field can use a part (for example,one state) of the states that can be characterized individually forSU-MIMO transmission. When the Group ID is specified for SU-MIMOtransmission, all the VHT STAs can demodulate and decode thecorresponding PPDU in the form for the SU-MIMO transmission and transmitthe corresponding data to their MAC layers without differentiating theSTAs from each other belonging to the same group for the MU-MIMOtransmission.

N_(SS-Field) ^(g) (SSAID) is a field value indicating spatial streams ofthe STAs involved for the MU-MIMO transmission when a Group ID # g hasbeen received. To be more specific, Equation 1 can be applied to thiscase:N _(SS-Field) ^(g) =M ⁰ ·N _(SS) ⁰ +M ¹ ·N _(SS) ¹ + . . . +M ^(N)^(MU−STA) ⁻¹ ·N _(SS) ^(N) ^(MU−STA) ⁻¹, where N _(SS) ^(k)∈{0,1,2, . .. ,M−1},  [Equation 1]

Where N_(SS) ^(k) is a variable indicating the number of spatial streamsof the k-th STA belonging to the Group ID # g.

For the convenience of description below, it is assumed that the maximumnumber of spatial streams that each individual STA can received duringMU-MIMO transmission is limited to 4 and thus a data service based onthe MU-MIMO transmission can be provided simultaneously for up to fourSTAs. Also, it is assumed that the number of spatial streams (SS) thateach individual STA can deal with is 1, 2, 3, or 4. Further, it isassumed that the maximum number of spatial streams that can betransmitted through one AP is 8.

At this time, if the value of N_(SS-Field) ^(g) transmitted by the AP is2+4×0+16×3+64×1=114, the number of SSs corresponding to a first STA inthe Group ID # g is 3, the number of SSs corresponding to a second STAis 1, and the number of SSs corresponding to a third STA is 4. AlthoughN_(SS-Field) ^(g) was transmitted to the last, fourth STA as if two SSswere assigned to the STA, since the total number of SSs used by theprevious three STAs amounts to eight, the number of SSs that can beassigned to the fourth STA is zero.

As another specific embodiment, if the value of N_(SS-Field) ^(g)transmitted by an AP is 3+5×1+25×2+125×0=58 under the same conditions,the number of SSs corresponding to a first STA in the Group ID # g is 3,the number of SSs corresponding to a second STA is 1, and the number ofSSs corresponding to a third STA is 3. Now, the number of SSs that canbe assigned to the last, fourth STA is zero.

According to the embodiment of the present invention, a signal can betransmitted such that by characterizing N_(SS-Field) ^(g) value, aparticular STA is allowed to use zero spatial streams. In this case, theAPs can transmit data through the MU-MIMO transmission by using asmaller number of SSs than the maximum allowed number of SSs. Inaddition, depending on the needs, by controlling the number of STAsreceiving a service simultaneously through particular MU-MIMOtransmission and the SS that each individual STA can receive in aflexible way, the APs can deal with the SU-MIMO and the MU-MIMOtransmission in an optimized way. Also, data transmission can be carriedout by distinguishing the SU-MIMO from the MU-MIMO by using the GroupID. The SSAIF can always assume a STA pairing requiring two or more STAsand this property can be used to compress the SSAIF information.

As another implementation example, spatial streams can also be mapped tothe individual STAs that are indicated by the respective Group IDs inthe form of a table as shown in Table 1.

TABLE 1 index SSAID #0 SSAID #1 SSAID #2 SSAID #3 0 0 0 1 1 1 0 0 1 2 20 0 1 3 3 0 0 1 4 4 0 0 2 2 5 0 0 2 3 6 0 0 2 4 7 0 0 3 3 8 0 0 3 4 9 00 4 4 10 0 1 1 1 11 0 1 1 2 12 0 1 1 3 13 0 1 1 4 14 0 1 2 2 15 0 1 2 316 0 1 2 4 17 0 1 3 3 18 0 1 3 4 19 0 2 2 2 20 0 2 2 3 21 0 2 2 4 22 0 23 3 23 1 1 1 1 24 1 1 1 2 25 1 1 1 3 26 1 1 1 4 27 1 1 2 2 28 1 1 2 3 291 1 2 4 30 1 1 3 3 31 1 2 2 2 32 1 2 2 3 33 2 2 2 2

Table 1 illustrates examples of the index values and the number ofspatial streams used for the respective STAs corresponding to thevalues; matching between the indices and the number of spatial streamsused for the respective STAs can be utilized while being modified byvarious combinations.

By using a total of 34 states in the SSAIF, a maximum of four spatialstreams available can be indicated for each individual STA; at the sametime, a maximum of eight spatial streams can be signaled. By using partof the information of the SSAIF or by making use of another informationfield, the order of permuting the STAs can be expressed in the Tableabove. For example, if the order of the STAs in the Table above isA-B-C-D, the order of A-C-B-D is equally possible; in addition, a totalof 24 ways of representing the order of the STAs can be realized.

If a logical association has been established only for STAs from amongthe Group ID, the SSAIF can express 816 states, namely, multiplicationof 34 states representing the number of spatial streams and a total of24 combinations for the order of the STAs, which can be expressed with10 bits. Alternatively, it is possible that the 34 states representingthe number of spatial streams are expressed with six bits and the 24states indicating the arrangement of the order of the STAs that informof the number of spatial streams are expressed with five bits, amountingto 11 bits in total. In the embodiment of the present invention, thetotal number of spatial streams is limited to eight to compress theSSAIF information and additionally, in the case of MU-MIMO transmission,the number of spatial streams that can be assigned to each individualSTA is limited to four.

Besides, the MU-MIMO transmission can be applied by further restrictingthe number of spatial streams available for each individual STA shown inTable 1. More specifically, by limiting the combination of the STAs thatcan be expressed in Table 1 and the corresponding spatial streams, theinformation to be transmitted can be further compressed. For example, ifthe maximum number of spatial streams available for each individual STAis limited to 2 and a new table is constructed, it is possible to make atable as shown in Table 2. The indices of Table 2 and the number ofspatial streams assigned to the STAs corresponding to the indices havebeen introduced for an illustrative purpose; the relationship can bechanged by various combinations and the maximum number of spatialstreams available for a single STA can also be changed.

TABLE 2 index SSAID#0 SSAID#1 SSAID#2 SSAID#3 0 0 0 1 1 1 0 0 1 2 2 0 02 2 3 0 1 1 1 4 0 1 1 2 5 0 1 2 2 6 0 2 2 2 7 1 1 1 1 8 1 1 1 2 9 1 1 22 10 1 2 2 2 11 2 2 2 2

If one takes account of the fact that data are transmitted to at leasttwo or more STAs in the MU-MIMO transmission and the maximum number ofspatial streams that can be transmitted by an AP, a total of 338 statesonly are required for the SSAIF. In other words, 9 bits (which canexpress 512 states) are required to express the whole information.

In Table 3, each STA can support a maximum of 4 spatial streams; Table4-3 illustrates SSAIDs that can be expressed when a maximum of 8 spatialstreams are allowed to be transmitted. When an actual system is supposedto be implemented, the corresponding field indices can be permuted indifferent ways.

TABLE 3 Field index SSAID#0 SSAID#1 SSAID#2 SSAID#3 0 0 0 1 1 1 0 0 1 22 0 0 1 3 3 0 0 1 4 4 0 0 2 1 5 0 0 2 2 6 0 0 2 3 7 0 0 2 4 8 0 0 3 1 90 0 3 2 10 0 0 3 3 11 0 0 3 4 12 0 0 4 1 13 0 0 4 2 14 0 0 4 3 15 0 0 44 16 0 1 0 1 17 0 1 0 2 18 0 1 0 3 19 0 1 0 4 20 0 1 1 0 21 0 1 1 1 22 01 1 2 23 0 1 1 3 24 0 1 1 4 25 0 1 2 0 26 0 1 2 1 27 0 1 2 2 28 0 1 2 329 0 1 2 4 30 0 1 3 0 31 0 1 3 1 32 0 1 3 2 33 0 1 3 3 34 0 1 3 4 35 0 14 0 36 0 1 4 1 37 0 1 4 2 38 0 1 4 3 39 0 2 0 1 40 0 2 0 2 41 0 2 0 3 420 2 0 4 43 0 2 1 0 44 0 2 1 1 45 0 2 1 2 46 0 2 1 3 47 0 2 1 4 48 0 2 20 49 0 2 2 1 50 0 2 2 2 51 0 2 2 3 52 0 2 2 4 53 0 2 3 0 54 0 2 3 1 55 02 3 2 56 0 2 3 3 57 0 2 4 0 58 0 2 4 1 59 0 2 4 2 60 0 3 0 1 61 0 3 0 262 0 3 0 3 63 0 3 0 4 64 0 3 1 0 65 0 3 1 1 66 0 3 1 2 67 0 3 1 3 68 0 31 4 69 0 3 2 0 70 0 3 2 1 71 0 3 2 2 72 0 3 2 3 73 0 3 3 0 74 0 3 3 1 750 3 3 2 76 0 3 4 0 77 0 3 4 1 78 0 4 0 1 79 0 4 0 2 80 0 4 0 3 81 0 4 04 82 0 4 1 0 83 0 4 1 1 84 0 4 1 2 85 0 4 1 3 86 0 4 2 0 87 0 4 2 1 88 04 2 2 89 0 4 3 0 90 0 4 3 1 91 0 4 4 0 92 1 0 0 1 93 1 0 0 2 94 1 0 0 395 1 0 0 4 96 1 0 1 0 97 1 0 1 1 98 1 0 1 2 99 1 0 1 3 100 1 0 1 4 101 10 2 0 102 1 0 2 1 103 1 0 2 2 104 1 0 2 3 105 1 0 2 4 106 1 0 3 0 107 10 3 1 108 1 0 3 2 109 1 0 3 3 110 1 0 3 4 111 1 0 4 0 112 1 0 4 1 113 10 4 2 114 1 0 4 3 115 1 1 0 0 116 1 1 0 1 117 1 1 0 2 118 1 1 0 3 119 11 0 4 120 1 1 1 0 121 1 1 1 1 122 1 1 1 2 123 1 1 1 3 124 1 1 1 4 125 11 2 0 126 1 1 2 1 127 1 1 2 2 128 1 1 2 3 129 1 1 2 4 130 1 1 3 0 131 11 3 1 132 1 1 3 2 133 1 1 3 3 134 1 1 4 0 135 1 1 4 1 136 1 1 4 2 137 12 0 0 138 1 2 0 1 139 1 2 0 2 140 1 2 0 3 141 1 2 0 4 142 1 2 1 0 143 12 1 1 144 1 2 1 2 145 1 2 1 3 146 1 2 1 4 147 1 2 2 0 148 1 2 2 1 149 12 2 2 150 1 2 2 3 151 1 2 3 0 152 1 2 3 1 153 1 2 3 2 154 1 2 4 0 155 12 4 1 156 1 3 0 0 157 1 3 0 1 158 1 3 0 2 159 1 3 0 3 160 1 3 0 4 161 13 1 0 162 1 3 1 1 163 1 3 1 2 164 1 3 1 3 165 1 3 2 0 166 1 3 2 1 167 13 2 2 168 1 3 3 0 169 1 3 3 1 170 1 3 4 0 171 1 4 0 0 172 1 4 0 1 173 14 0 2 174 1 4 0 3 175 1 4 1 0 176 1 4 1 1 177 1 4 1 2 178 1 4 2 0 179 14 2 1 180 1 4 3 0 181 2 0 0 1 182 2 0 0 2 183 2 0 0 3 184 2 0 0 4 185 20 1 0 186 2 0 1 1 187 2 0 1 2 188 2 0 1 3 189 2 0 1 4 190 2 0 2 0 191 20 2 1 192 2 0 2 2 193 2 0 2 3 194 2 0 2 4 195 2 0 3 0 196 2 0 3 1 197 20 3 2 198 2 0 3 3 199 2 0 4 0 200 2 0 4 1 201 2 0 4 2 202 2 1 0 0 203 21 0 1 204 2 1 0 2 205 2 1 0 3 206 2 1 0 4 207 2 1 1 0 208 2 1 1 1 209 21 1 2 210 2 1 1 3 211 2 1 1 4 212 2 1 2 0 213 2 1 2 1 214 2 1 2 2 215 21 2 3 216 2 1 3 0 217 2 1 3 1 218 2 1 3 2 219 2 1 4 0 220 2 1 4 1 221 22 0 0 222 2 2 0 1 223 2 2 0 2 224 2 2 0 3 225 2 2 0 4 226 2 2 1 0 227 22 1 1 228 2 2 1 2 229 2 2 1 3 230 2 2 2 0 231 2 2 2 1 232 2 2 2 2 233 22 3 0 234 2 2 3 1 235 2 2 4 0 236 2 3 0 0 237 2 3 0 1 238 2 3 0 2 239 23 0 3 240 2 3 1 0 241 2 3 1 1 242 2 3 1 2 243 2 3 2 0 244 2 3 2 1 245 23 3 0 246 2 4 0 0 247 2 4 0 1 248 2 4 0 2 249 2 4 1 0 250 2 4 1 1 251 24 2 0 252 3 0 0 1 253 3 0 0 2 254 3 0 0 3 255 3 0 0 4 256 3 0 1 0 257 30 1 1 258 3 0 1 2 259 3 0 1 3 260 3 0 1 4 261 3 0 2 0 262 3 0 2 1 263 30 2 2 264 3 0 2 3 265 3 0 3 0 266 3 0 3 1 267 3 0 3 2 268 3 0 4 0 269 30 4 1 270 3 1 0 0 271 3 1 0 1 272 3 1 0 2 273 3 1 0 3 274 3 1 0 4 275 31 1 0 276 3 1 1 1 277 3 1 1 2 278 3 1 1 3 279 3 1 2 0 280 3 1 2 1 281 31 2 2 282 3 1 3 0 283 3 1 3 1 284 3 1 4 0 285 3 2 0 0 286 3 2 0 1 287 32 0 2 288 3 2 0 3 289 3 2 1 0 290 3 2 1 1 291 3 2 1 2 292 3 2 2 0 293 32 2 1 294 3 2 3 0 295 3 3 0 0 296 3 3 0 1 297 3 3 0 2 298 3 3 1 0 299 33 1 1 300 3 3 2 0 301 3 4 0 0 302 3 4 0 1 303 3 4 1 0 304 4 0 0 1 305 40 0 2 306 4 0 0 3 307 4 0 0 4 308 4 0 1 0 309 4 0 1 1 310 4 0 1 2 311 40 1 3 312 4 0 2 0 313 4 0 2 1 314 4 0 2 2 315 4 0 3 0 316 4 0 3 1 317 40 4 0 318 4 1 0 0 319 4 1 0 1 320 4 1 0 2 321 4 1 0 3 322 4 1 1 0 323 41 1 1 324 4 1 1 2 325 4 1 2 0 326 4 1 2 1 327 4 1 3 0 328 4 2 0 0 329 42 0 1 330 4 2 0 2 331 4 2 1 0 332 4 2 1 1 333 4 2 2 0 334 4 3 0 0 335 43 0 1 336 4 3 1 0 337 4 4 0 0

In Table 4, each STA can support a maximum of 2 spatial streams; Table 4illustrates SSAIDs that can be expressed when a maximum of 8 spatialstreams are allowed to be transmitted. When an actual system is supposedto be implemented, the corresponding field indices can be permuted indifferent ways.

TABLE 4 Field index SSAID#0 SSAID#1 SSAID#2 SSAID#3 0 0 0 1 1 1 0 0 1 22 0 0 2 1 3 0 0 2 2 4 0 1 0 1 5 0 1 0 2 6 0 1 1 0 7 0 1 1 1 8 0 1 1 2 90 1 2 0 10 0 1 2 1 11 0 1 2 2 12 0 2 0 1 13 0 2 0 2 14 0 2 1 0 15 0 2 11 16 0 2 1 2 17 0 2 2 0 18 0 2 2 1 19 0 2 2 2 20 1 0 0 1 21 1 0 0 2 22 10 1 0 23 1 0 1 1 24 1 0 1 2 25 1 0 2 0 26 1 0 2 1 27 1 0 2 2 28 1 1 0 029 1 1 0 1 30 1 1 0 2 31 1 1 1 0 32 1 1 1 1 33 1 1 1 2 34 1 1 2 0 35 1 12 1 36 1 1 2 2 37 1 2 0 0 38 1 2 0 1 39 1 2 0 2 40 1 2 1 0 41 1 2 1 1 421 2 1 2 43 1 2 2 0 44 1 2 2 1 45 1 2 2 2 46 2 0 0 1 47 2 0 0 2 48 2 0 10 49 2 0 1 1 50 2 0 1 2 51 2 0 2 0 52 2 0 2 1 53 2 0 2 2 54 2 1 0 0 55 21 0 1 56 2 1 0 2 57 2 1 1 0 58 2 1 1 1 59 2 1 1 2 60 2 1 2 0 61 2 1 2 162 2 1 2 2 63 2 2 0 0 64 2 2 0 1 65 2 2 0 2 66 2 2 1 0 67 2 2 1 1 68 2 21 2 69 2 2 2 0 70 2 2 2 1 71 2 2 2 2

Meanwhile, in the case of downlink MU-MIMO transmission, when the numberof STAs receiving a service from the AP is large and the combination ofSTAs that can be indicated by the Group ID is restricted a lot, usingall of the Group IDs can be inefficient. In particular, if thesleep/wake state of each individual STA is not fully synchronized witheach other while the STAs operating in a Power Saving Mode are groupedand logically connected to each other by a group ID, the AP shouldchange either the group by using a management PPDU frame or theconfiguration of the power saving mode.

Therefore, the present invention additionally provides a method foroperating STAs separately from a group ID. As shown in Tables 1 and 2,in order for a particular, virtual STA to obtain the number of spatialstreams, each individual STA informed of its STA number ID determinedwithin the corresponding group through the Group ID. Separate from theGroup ID, each STA can determine the corresponding number ID through amanagement frame or predetermination. In the present invention, the STAnumber ID is called SSAID. The SSAID represents the position of a streamamong the streams serviced at the same time by an AP, which should bereceived by STAs. For example, if STA1, STA2, STA3, and STA4 correspondrespectively to SSAID 1, 2, 3, and 4, each individual STA receives afirst package of streams, a second package of streams, a third packageof streams, and a fourth package of streams from among a plurality ofstreams serviced by the AP. In other words, all of the STAs have asingle SSAID. If the maximum number of STAs serviced at the same time isN, the value of the SSAID can range from 1 to N.

According to another embodiment of the present invention, the SSAID ofeach STA can be indicated through the management frame, but the SSAIDcan also be indicated through a predetermined rule. For example, theSSAID can be mapped to a function of the association ID assigned duringthe process where an STA establishes an association with an AP fortransmitting and receiving data. As a more specific example, a module Nvalue of the association ID can be used for the SSAID (it is assumedthat the value of the SSAID ranges from 0 to N−1). In addition, each STAis informed of a Group ID and the order of the STA in the correspondingGroup by following the Group ID scheme; at the same time, the STA can beinformed of the number of spatial streams by using the SSAID assignedpreviously separately from the Group ID.

For example, if the Group ID field corresponds to a particular state(i.e., index 15 in the case of transmitting a 4-bit Group ID), the SSAIDis determined as the order of the STAs connected logically with the sameGroup ID and the STAs in the corresponding group are determined. If theGroup ID field corresponds to a different state, each spatial stream isreceived by using the SSAID predetermined or assigned irrespective ofthe Group ID. In the latter case, multiple STAs can occupy the sameSSAID and all of the multiple STAs can carry out decoding thecombination of particular spatial streams.

To implement downlink MU-MIMO transmission, the VHT SIG field of thePLCP header includes a group ID and the MU-MIMO set of spatial streamsub-field. The reserved Group ID 16 is used for the downlink MU-MIMOtransmission. The downlink MU-MIMO transmission is carried out for thoseSTAs not belonging to a group. At this time, each individual STAreceives the corresponding downlink (DL) MU-MIMO transmission data basedon its SSAID.

The MU-MIMO Set of Spatial Streams sub-field is a rotated sequencecomprising 0s and 1s, informing each individual STA of the number ofspatial streams assigned. For example, in the case of 0000 1111 00001111 0000, it indicates that four spatial streams have been assigned toeach of the 1st STA, the 2nd STA, the 3rd STA, and the 4th STA. In thiscase, the 1st STA indicates those terminals with the SSAID of 1. The 2ndSTA indicates those terminals with the SSAID of 2; the 3rd STA thoseterminals with the SSAID of 3; the 4th STA those terminals with theSSAID of 4. For example, STA A and STA B enter the wakeup mode whileoperating in the power saving mode; and STA A and STA B are not groupedyet. However, the SSAID value has already been assigned to all the STAsby an AP. It is assumed that the SSAID of STA A is 1; the SSAID of STA Band STA C is 2; and the SSAID of STA D is 4. It is also assumed that theAP has transmitted eight spatial streams to each of the STA A and STA B.In this case, the group ID is 16 and the MU-MIMO Set of Spatial Streamsfield is set to 0000 0000 1111 1111, comprising the PLCP header.

Each of the STAs, if the group ID corresponds to a particular state(e.g., index 15), regards the particular state as applying to itself(this behavior may not apply to all the STAs but only to the STAs notgrouped into a group). The STA A also regards the downlink MU-MIMOtransmission data as applying to itself and performs channel estimationthrough a 1st LTF sequence set. (It is because the SSAID of STA A is 1.)The STA B and STA C also regard the downlink MU-MIMO transmission datajust received as applying to themselves, performing channel estimationthrough a 2nd LTF sequence set. (This is because the SSAID of the STA Band the STA C is 2.) However, in this case, since the STA C is not atarget terminal of the downlink MU-MIMO transmission, the STA C is asgood as overhearing. Since the above example assumed two STAs, the 4thLTF sequence set is not needed and thus, the STA D does not perform thetask of channel estimation. (Since the SSAID of the STA D is 4, the STAD considers the 4th LTF sequence set as the channel information foritself.) Also, the STA C cannot detect the LTF sequence set directed tothe STA B due to pre-coding. In this case, too, the STA C realizes thatthe current DL MU-MIMO transmission is not intended for itself and stopsthe overhearing.

According to another embodiment of the present invention, a group ofSTAs can be operated in the MAC layer to support MU-MIMO transmission ofa large number of STAs. In particular, the present invention operates agroup of STAs by using an AP, but operates a group indicated in the PHYlayer as a subset of a group indicated in the MAC layer.

In the MU-MIMO transmission, STAs are grouped into a particular groupand a particular Group index is assigned to the group; the STAs areinformed of the Group index so that which particular STAs are scheduledsimultaneously to participate in the MU-MIMO transmission for receivingdata. In general, the number of groups that can be operated should belarge enough to support a combination of a large number of STAs. Thecombination of particular STAs should be informed of through the PHYlayer so that the corresponding, data-receiving STAs determines whetherto receive data and based on the determination result, receives datapackets by demodulating and decoding particular spatial streams.However, delivering a large amount of information through the PHY layercauses large signaling overhead and a related protocol to support theoverhead can be complicated.

According to an embodiment of the present invention, to solve the aboveproblem, a group hierarchy can be constructed. The information signaledin the PHY layer corresponds to sets of STA grouping, while theinformation signaled in the MAC layer the final sets of STA grouping. Atthis time, the information signaled in the PHY layer can be transmittedto the PLCP header such as VHT-SIG, while the information signaled inthe MAC layer can be transmitted in the MAC layer in the form of datapackets.

FIG. 12 illustrates one example of indicating STA group sets in the PHYlayer and indicating STA groups in the MAC layer according to anembodiment of the present invention.

When a group set is indicated in the PHY layer, the group set canindicate a group of a plurality of STAs. For example, if the informationin the PHY layer indicates Group Set #1, the corresponding Group Set mayinclude Group #1, #2, #3, and #4. Each individual group represents a setof particular STAs. As one example, each of the corresponding groups canhave a set of STAs as shown in Table 5. At this time, A, B, C, D, E, F,G, and H represents STAs different from each other.

TABLE 5 GROUP STA Group #1 A, B, C, G Group #2 A, B, F, D Group #3 A, E,C, D Group #4 H, E, F, G

In general, each STA can belong to multiple groups; to reduce thecomplexity imposed on the STA that receives data, it is preferred thatthe order of the STA among the multiple groups to which the STA belongsshould be informed of.

If groups are already defined, the AP transmits data through the MU-MIMOtransmission by indicating a Group Set index in the VHT-SIG of the PLCPheader of the PPDU frame through which the data are transmitted; theGroup Set index indicated in the VHT-SIG sometimes corresponds to aplurality of groups. While a plurality of groups are indicated, aplurality of STAs may decode a particular set of spatial streams as ifthey correspond to the spatial streams intended for the STAs; in thiscase, whether the data packets are associated with the STAs or not canbe known from the MAC ID in the MAC layer.

FIG. 13 is one example of frame transmission by using Group Setindication signaled in the PHY layer and Group indication signaled inthe MAC layer.

There are Set ID 1 and 2; two groups are defined for each Set ID; and aset of STAs is defined for each group as shown in FIG. 13. If the Set ID1 is indicated by the VHT-SIG, all the STAs corresponding to Set ID=1attempt decoding; eventually, the STAs belonging to each group succeedsin decoding a set of the corresponding spatial streams and based on theMAC ID, transmits the data packets to an upper layer.

In other words, the present embodiment is similar to a method ofincreasing the number of STAs that can be supported in the MU-MIMOtransmission by associating the group identity with multiple sets ofSTAs rather than associating the group identity mentioned above with oneset of particular STAs. However, the present invention operates a groupof STAs in the PHY and MAC layer to reduce sounding and the complexityin the various MAC protocols.

FIG. 14 illustrates a format of the management information included inthe management frame according to an embodiment of the presentinvention. To transmit data packets by using Group Set indicationsignaled in the PHY layer and Group indication signaled in the MAClayer, the management information of FIG. 14 can be transmitted throughdata packets that can be signaled to the management frame or the STAs.In the example of FIG. 14, the Group Set ID corresponds to theidentifier of groups managed in the PHY layer, while the Group ID a setof particular STAs managed in the MAC layer.

It should be noted that if the Group Set identity is managed in the PHYlayer and groups are managed in the MAC layer, the MU-MIMO transmissioncan be implemented with little overhead in terms of the soundingprotocol that carries out CSI feedback.

For example, if the AP attempts CSI feedback (sounding) only for theSTAs belonging to a particular group, only the STAs associated with thecorresponding group are allowed for the CSI feedback (sounding) bysignaling the Group ID defined in the MAC layer.

FIG. 15 is an example where a method of indicating a group by using MAClayer and PHY layer is applied to data packet transmission.

As one embodiment, the present invention manages the Group Setidentifier transmitted from the PHY layer by using 4 bits, while theGroup identifier is managed by using 8 bits to deal with a lot moreactual groups in the MAC layer. The 8 bit Group identifier in the MAClayer can support a maximum of 256 sets of STAs, giving the AP theflexibility of scheduling approximately 10 STAs without restriction.

In a method of indicating a particular group in the MAC layer when aplurality of groups are operated after being associated with particulargroup sets, the control information can be transmitted to the VHT-SIG ofthe PLCP header part to be transmitted. At this time, the VHT-SIG can betransmitted while being divided into VHT-SIG-A and VHT-SIG-B. The GroupSet information is transmitted first through the VHT-SIG-A and indicatesthe sets of terminals to perform decoding; and the VHT-SIG-B indicates aparticular group such that which terminal should receive thecorresponding spatial stream. The VHT-SIG-B can indicate exactly whichSTAs should receive the MU-MIMO transmission and indicate the ordernumber of a group among the Group Set. In addition, the VHT-SIG-Btransmitted separately for each individual STA can indicate thecorresponding STA exactly by transmitting an ID with which an STA can beidentified. At this time, the ID for identifying an STA can correspondto the Association ID.

FIG. 16 is a block diagram illustrating a transmitter in which oneembodiment of the present invention is implemented. The transmitter 1600can be an AP or a non-AP STA.

The transmitter 1600 comprises a processor 1610, a memory 1620, a radiofrequency (RF) unit 1630, and a multiple antenna 1650. The RF unit 1630is configured to transmit the management frame of the present inventionand data packets, the processor 1610, connected to the RF unit 1630, isconfigured to generate and process the management frame and datapackets. The processor 1610 and the RF unit 1630 implements the physicallayer and the MAC layer of IEEE 802.11 specifications. The processor1610 and/or the RF unit 1630 may include ASIC (Application-SpecificIntegrated Circuit), other chipset, a logic circuit and/or dataprocessing apparatus. The memory 1620 may include ROM (Read-OnlyMemory), RAM (Random Access Memory), flash memory, a memory card, astorage medium, and/or other storage device. If an embodiment isimplemented by software, the technique described above can beimplemented as a module (a process, a function, and so on) performingthe aforementioned function. The module can be stored in the memory 1620and can be executed by the processor 1610. The memory 1620 can bepositioned inside or outside of the processor 1610; and can be connectedto the processor 1610 through various well-known means.

In a wireless LAN system supporting MU-MIMO transmission, the presentinvention can efficiently indicate the destination STA of the MU-MIMOtransmission and the spatial stream to be received, by the destinationSTA, by using transmission of little amount of information.

The embodiments described above include various types of examples.Though it may not be possible to describe all the possible combinationsfor illustrating the various types, those skilled in the art wouldunderstand that other combinations are possible. Therefore, it should beunderstood that the present invention includes all the othersubstitutions, modifications, and changes belonging to the scope asdefined by the appended claims.

What is claimed is:
 1. A method used in a wireless local area network, the method performed by a receiving station and comprising; receiving a Very High Throughput (VHT) physical layer protocol data unit (PPDU) including a group identity (ID) field, a spatial stream (SS) field and a data field, wherein the group ID field and the SS field are included in a VHT signal (SIG) field of the VHT PPDU, wherein the SS field includes information on a total number of SSs used by the receiving station, wherein the group ID field has a length of M bits representing 2^(M) different states, each of the 2^(M) different states including information for either a single user-multiple input multiple output (SU-MIMO) scheme used for the VHT PPDU or a group ID for a group to which the receiving station belongs, and wherein only one state of the 2^(M) different states is used for the VHT PPDU being received from an access point (AP) based on the SU-MIMO scheme; and decoding the data field based on the SS field.
 2. The method of claim 1, wherein the VHT SIG field belongs to a physical layer preamble of the VHT PPDU.
 3. The method of claim 1, wherein the receiving station is a non-AP station.
 4. A receiving station for receiving a data packet in a wireless local area network, the receiving station comprising; a transceiver configured to transmit and receive radio signals; and a processor operably coupled to the transceiver and configured to: control the transceiver to receive a Very High Throughput (VHT) physical layer protocol data unit (PPDU) including a group identity (ID) field, a spatial stream (SS) field and a data field, wherein the group ID field and the SS field are included in a VHT signal (SIG) field of the VHT PPDU, wherein the SS field includes information on a total number of SSs used by the receiving station, wherein the group ID field has a length of M bits representing 2^(M) different states, each of the 2^(M) different states including information for either a single user-multiple input multiple output (SU-MIMO) scheme used for the VHT PPDU or a group ID for a group to which the receiving station belongs, and wherein only one state of the 2^(M) different states is used for the VHT PPDU being received from an access point (AP) based on the SU-MIMO scheme; and decode the data field based on the at least one SS field.
 5. The receiving station of claim 4, wherein the VHT SIG field belongs to a physical layer preamble of the VHT PPDU.
 6. The receiving station of claim 4, wherein the receiving station is a non-AP station. 